Method of protecting color chart in determination of chromate concentrations



Patented July 29, 1 952 UNITED sTA i-as PATENT OFFICE I faeosllee METHODor. PROTECTING COLOR CHART IN DETERMINATION OF CHROMATE CONCENTRATIONSIiouisEarlOdelh. Chicago, 111., assignor to Dearborn Ghemicali-Company,Chicago, 111., a cor- "pcration of Illinois.

I No Drawing Application, January 21, 1949,

The present invention relates to a method of protecting a color chartfor the determination of chromate concentrations and moreparticularly-relatestoa method of'improv-ingthe-[storage characteristicsof such {color charts prepared by'theimpregnation of bibulous mediawith-benzidine and acid'd'erivatives of-benzidine.

It has heretofore been proposed to employ its colored. or quinoid-forma.Such "compositions maybe used inthe quemtitative determinatin ofchromate concentrations by a simple spot. test, sinceav sharp colorbreak is. obtained upon the exposure of the impregnated bibulousmaterial to an aqueous solution of a water soluble chromate inconcentrations such as are used in the cooling systems of. internalcombustion engines to inhibit the formation ofrust therein;

I have now found that testing charts formed of bibulous materialimpregnated with the testing solution described above deteriorates uponstanding, probably due to oxidation in the presence of moisture, sothat, upon testing, no sharp color break is observed. Instead of thissharp color break, a gradual color change will: be noted uponan-increase in concentration ofthe chromate solution and itis impossibleaccurately to judge. the concentration'of'thesolution under.-goingtesting; by this method;

In order to prevent the deterioration of the testing media in the manneras hereinbefore described, and to increase the stora e life of thetesting media, I have found that it is desirable tostore the analytical.compositions in the presence otasubstantiallydry atmosphere. Thisstorage .of thetesting. media may be. carried out. inany desired andsuitable-manner, as, for example, by storing the media in the. presenceof a desiccating agent, by storage in a vacuum, by,

storage in a closed container in an atmosphere of aninert dry gas, andin other ways well known to those skilled in the art.

By this storage in' a dry atmosphere, it is possible to maintain thetesting media in effective condition for an extended length of time.

Thus, it isnot necessary to have on. hand freshly prepared testing mediaat all times, nor is it necessary to underwrite the expense ofdiscarding testing media after their short life of I Serial'No. 72,055.a claims; (01. 23-1) usefulness. has expired, vashas been heretoforenecessary.

It is therefore an important object oi. the

present invention to provide. a method for'the protection ofoxidation-sensitive chromate testing materials b the storage of thematerial in a dry atmosphere.

7 It is another important object of the present invention to provide acontainer. for the storage of testing media, including a bibulousmaterial impregnated with benzidine, or, acid] derivative thereof, thecontainerbeingmoistureeproof and having means. for drying. the interior.thereof;

It is a further important object ofthe present invention toprovideaimethod for prolonging the.

storage. life of testing media, such as bibulous material. impregnatedwith a solution of benzi- 1 250. ml. water 3 dine or a derivative. ofbenzidine 'and an organicacid for the testing ofil aqueous. chromatesolutions by color developed in. thevmedia, which comprises storing themedia in a closedcontainer in the presence of a dry atmosphere topreserve the color sensitivityof the medi 7 I Qtherand further objectsofthis invention willv become apparentfrom the following description andappended claims.

The testing medium employedv is a dry-bibulous sheet material. such as asheet of blotting paper, impregnated with a solution containingbenzidineor an acid derivative of benzidine-and also, op.- tionally, containinganorganic crystalline acid and, a small quantityof starch.Typicalana-lytical compositions maybe prepared by immersing.

separate absorbent paper blotters-in anyof the following compositions-z1 g. benzidine dihydrochloride 20 g. citricv acid I g 0.5 g. corn starch250-ml. water" p I III 1. g.. benzidine di-actaLte 20 g. citric acid0,5. g.. corn starch I V l g. benzidine 20 g..citric acid 0.5 g. cornstarch 250 ml. ethyl alcohol In the literature, the sensitivity. ofbenzidine and? its derivatives to light. and oxidation has'beendescribed. However, "I have'f'ound that thei'ana :lytical compositionsabove described are re1a-'1 3 tively non-sensitive to light andoxidation, if kept away from moisture, but are definitely sensitive tooxidation in the presence of moisture. N ap-' preciable color break canbe observed with theuse of these compositions for testing chromatesolutions following storage for as short a time as two or three weeks ifleft exposed to oxidation under normal relative humidity conditions in aroom.

This sensitivity to oxidation will be readily understood from thefollowing experimental data: I Erperimentl Testing material for use inthetesting of aqueous chromate concentrations of aqueous chromatesolutions was prepared as follows:

0.5 g. soluble starch was placed in 250 ml. of distilled water andheated to boiling. The starch solution was cooled slightly and pouredover one gram of benzidine di-hydrochloride and 20 grams of citric acid.Absorbent paper blotters were soaked in the resulting solution, whencooled, for. thirty seconds, and then were allowed to drain r verticallyfor thirty seconds. They were then placed horizontally and allowed todry at 77 F.

(relative humidity 50 The cards as .prepared'above, when promptly used,showeda color break of fromdeep blue to golden with greenish-blue flecksin the presence of a'chromate solution containing two pounds of standardrust inhibiting solution, containing 62.5 parts by weight of sodiumdichromate and 30 parts by weight of sodium carbonate in'a total of llo'parts by weight 'of-dry' composition, per 100 gallons of water. Afterstorage for a period of from sevenito 'nine'weeks at atmosphericconditions, no sharp color "break was obtained upon exposure tochromatesolutions of varying concentrations. V 7 V I have found that thedeterioration of such analytical compositions may be prevented if,promptly after being prepared, the compositions are stored in theabsence of moisture or, more particularly, in a dry atmosphere,regardless of whether oxygen is present in or is excluded from suchatmosphere.

This storage may be carried out in any desired manner so long as contactbetween the analytical compositions'and oxygen or oxidizing atmospheresin the'presence of moisture is prevented. As exemplary of Suitable meansof preventing this contact with moisture, the analytical compositionsmay be stored under a vacuum in conven tional desiccating apparatus inthe presence of a suitable desiccant such as calcium chlorideqthecompositions may be stored within 'a closed but unevacuated container inthe presence of a suitable 'desiccating agent such as calcium chloride 1r or silica gel; or the compositions may be stored within a closedcontainer containing any type of a suitable dried atmosphere. Ashereinbefore explained, it is not necessarythat the composition bemaintained out of contact with oxygen to pre 7 vent deterioration of theproperties of the composition, inasmuch as dry oxygen itself does notcause such 'deteriorationfl' Various simple containers, such as metalfoil, cellophaneror other. plastic 'membranes, or moisture-proof coated.

paper, maybe employed for the wrapping, so

long as some means is provided for preventing contact between thetesting compositions and moisture. For example, a suitable package maycomprise a waxed paper wrapping enclosing the analytical compositionsanda small body of silica The dryness of the atmosphere is important,although the exact moisture content of the atmosphere need not becontrolled within narrow limits.

Preferably, however, I have found that it is desirable to limit themoisture content of the atmosphere to that equivalent to an atmospherehaving a dewpoint of 50 F. or less while a dewpoint of -80? F. or lessis preferred for indefinite storage. Such anatmosphere may be readilyobtained by the employment of conventional desiccating agents. Forexample, it has been found that one ounce of silica gel disposed withina quart container, assuming that the air in the container is saturatedwith moisture at room temperature, will give a dewpoint of 80 F. whenstatic equilibrium is reached. Thus, it will readily be seen that myimproved method of enhancing the storage characteristicsof suchanalytical composition may be out.

some of the test results obtained by the use of various means formaintaining the analyticalcompositions prepared as hereinbeforedescribedin a dry atmosphere:

v Experiment II Cards prepared as described in connection withExperiment I, promptly after being prepared, were placed in a desiccatorcontaining calcium' chloride and the desiccator was evacuated. The

' desiccator was allowed to refillwith carbon dioxide and the evacuationand refilling with car bon dioxide wasrepeated three times. storageperiod of approximately four months, the

following test resultsw'ere obtained in the pres-1 ence of aqueoussolutions containing the indicated amount of a rust inhibitor compoundof equal parts of sodium dichromate and sodium carbonate.

easily, and readily carried.

The following} experimental data illustrates After a i Inhibitor Con-Length of Storage centration per Observed Color gal. H O I v 1 5 S1 ht1d I 1g go None "7":7" 5 glr oitileunegd gold.

' v j s 1g go 4 months" T 2 Pronounced gold;

' A control consisting of 'a card prepared in exactly the same mannerand stored under atmospheric conditions showed the development of nogold color when tested at the end of eight weeks. A similar control was"run in connection with each of the following'experiments and similar 7results were obtained Experiment III Cards prepared as describedinconnecticn with] Experiment I'were stored in ,a sealedbut un-fevacuated quart can containing the below spe The following.. re-

fied quantities of silica gel. sults were observed:

, j 7 Inhibitor Oona Length of Storage centration per Observed Color 100gal. H O f A llbs. 1 '1 None 1 ggg sh -V mes. 2 oz. silica gel)' 5ffggjgg'gg gfi 2 months (1 oz, silica gel) e V g 5 gold Example IV Thefollowing tests were conducted. to determine various types ofsuitablewrapping which may be employed in conjunction with a desiccant to storethe analytical compositions in a substantially dry atmosphere. Theanalytical compositions were prepared in the same manner as defined inExperiment I and the following results were obtained:

. 6 mosphere having a dew point at leastas low as 50F. v

2. The method of protecting analytical compositions comprising abibulous material impregnated with a reagent selected from the groupconsisting of benzidine and acid derivatives of benzidine whichcomprises promptly after preparation thereof disposing said compositionsin a closed container in the presence of a desiccant,

From the experimental data hereinbeiore given, it may be seen that thedeterioration of the analytical compositions is substantially completelyinhibited upon storage in the absence of moisture. The packaging of theanalytical composition within a simple wrapping of waxed paper I in thepresence of a desiccant is sufficient to prevent storage deteriorationeven through the compositions may be exposed to oxygen. The advantagesresiding in the method of the present invention and the improved resultsobtained 1 thereby will be evident t/o those skilled in the art.

It will, of course, be understood that various details of the processmay be varied through a wide range without departing from the principlesof this invention and it is, therefore, not the purpose to limit thepatent granted hereon otherwise than necessitated by the scope of theappended claims.

I claim as my invention:

1. The method of protecting analytical compositions comprising abibulous material impregnated with a reagent selected from the groupconsisting of benzidine and acid derivatives of benzidine whichcomprises promptly after preparation thereof storing and maintainingsaid compositions in a substantially moisture-free at- 25 said desiccantbeing present in a sufiicient amount to provide and maintain anatmosphere in said container having a dew point at least as low as -50F., and maintaining said compositions within said container until readyfor use.

LOUIS EARL ODELL.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS OTHER REFERENCES Mitchell, Recent Advances InAnalytical Chemistry, Published by Blakistons Son & Co., Inc.,

Philadelphia, Pa. 1931, page 129.

Flags, John R, Organic Reagents" Interscience Publishers Inc., New York1948, page 233.

1. THE METHOD OF PROTECTING ANALYTICAL COMPOSITIONS COMPRISING ABIBULOUS MATERIAL IMPREGNATED WITH A REAGENT SELECTED FROM THE GROUPCONSISTING OF BENZIDINE AND ACID DERIVATIVES OF BENZIDINE WHICHCOMPRISES PROMPTLY AFTER PREPARATION THEREOF STORING AND MAINTAININGSAID COMPOSITIONS IN A SUBSTANTIALLY MOISTURE-FREE ATMOSPHERE HAVING ADEW POINT AT LEAST AS LOW AS -50* F.